Setting starts and ends
As you maybe get it until now a DNA string is made of two single strings. The upper one is called
"5'-3' string", the other one "3'-5' string". These names have they got from their structure and their
anti-parallelity. (But this is too much biology! See Creating a complete DNA string
for further details.)
For cutting and copying you need to set a region you would like to work with. So you can
move the slider at the bottom to change your position and set the marks with this options.
In graphic mode marked regions appear cyan...
... in text mode these regions are underlined so that you can see them "better".